Oh Maria this is so sweet! And I actually think I did something similar to ask my bridesmaids! I made little felt envelopes with their names stitched on the outside and had something similar written inside. Except our nicknames for each other aren't appropriate to be written somewhere for the general public to see ;)

Really sweet and it's so lovely you have such a close relationship with your sister in law. No better person to be by your side as you're getting ready for your big day xx